AR-15

The AR-15 comes in many sizes and has many options, depending on the manufacturer. The part shown bottom center is the lower receiver with pistol grip and trigger assembly. Under U.S. law the receiver alone is the component legally considered the "firearm".

The AR-15 is a lightweight, 5.56 mm/.223-caliber, magazine-fed, air cooled rifle with a rotating-lock bolt, actuated by direct impingement gas operation or long/short stroke piston operation. It has been produced in many different versions, including numerous semi-automatic and selective fire variants. It is manufactured with extensive use of aluminum alloys and synthetic materials.

The AR-15 was first built by ArmaLite as a small arms rifle for the United States armed forces.[8] Because of financial problems, ArmaLite sold the AR-15 design to Colt. After modifications (most notably the relocation of the charging handle from under the carrying handle like the AR-10 to the rear of the receiver), the new redesigned rifle was subsequently adopted as the M16 rifle.[9] Colt then started selling the semi-automatic version of the M16 rifle as the Colt AR-15 for civilian sales in 1963 and the term has been used to refer to semiautomatic-only versions of the rifle since then.[10] Although the name "AR-15" remains a Colt registered trademark, variants of the firearm are independently made, modified and sold under various names by multiple manufacturers.

ArmaLite sold its rights to the AR-10 and AR-15 to Colt in 1959. After a tour by Colt of the Far East, the first sale of AR-15s was made to Malaya on September 30, 1959, with Colt's manufacture of their first 300 AR-15s in December 1959.[13] Colt marketed the AR-15 rifle to various military services around the world, including the U.S. Navy, Air Force, Army, and Marine Corps. The AR-15 was eventually adopted by the United States military under the designation M16. Colt continued to use the AR-15 trademark for its semi-automatic variants (AR-15, AR-15A2) which were marketed to civilian and law-enforcement customers. The original AR-15 was a very lightweight weapon, weighing less than 6 pounds with empty magazine. Later heavy-barrel versions of the civilian AR-15 can weigh upwards of 8.5 lb.[14]

Today the AR-15 and its variations are manufactured by many companies and are popular among civilian shooters and law enforcement forces around the world due to their accuracy and modularity (for more history on the development and evolution of the AR-15 and derivatives see M16 rifle).

The trademark "AR15" or "AR-15" is registered to Colt Industries, which maintains that the term should only be used to refer to their products. Other AR-15 manufacturers make AR-15 clones marketed under separate designations, although colloquially these are sometimes referred to by the term AR-15.

Semi-automatic AR-15s for sale to civilians are internally different from the full automatic M16, although nearly identical in external appearance. The hammer and trigger mechanisms are of a different design. The bolt carrier and internal lower receiver of semi-automatic versions are milled differently, so that the firing mechanisms are not interchangeable. This was done to satisfy United States Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) requirements that civilian weapons may not be easily convertible to full-automatic. In the late 1970s and early '80s, items such as the "Drop In Auto Sear" or "lightning-link," conversion to full automatic was very straightforward (sometimes requiring machining of the lower receiver with use of a mill and M16 Bolt Carrier Group).[15][16] Such modifications, unless using registered and transferable parts made prior to May 19, 1986, are illegal. (The Firearm Owners Protection Act in 1986 has redefined a machine gun to include individual components where a semi-automatic firearm can be converted to full-automatic based on a 1981 ATF ruling on machine gun parts.) Since 1993, The Bolt Carrier Groups used in AR-15 type rifles for civilians have employed additional measures to prevent modification to full auto. Colt AR-15's use a metal alloy wall separating the Fire Control group from the Sear, preventing use of such items.

Automatic variants have a three-position rotating selective fire switch, allowing the operator to select between three modes: safe, semi-automatic, and either automatic or three-round burst, depending on model. Civilian Colt AR-15 models do not have three-round burst or automatic settings; they can only be fired as a semi-automatic, and are therefore not selective fire weapons. In semi-automatic-only variants, the switch only functions to rotate between safe and fire. Some other manufacturers may mark their rifles with three-positions for collectors and re-enactors, though the guns will not fire in those modes. However weapons modified to full automatic using a lightning-link are capable of full automatic fire only—unless a special full automatic fire select mechanism and modified selector-switch is substituted.[15]

The main mechanism of operation for the rifle is known as direct gas impingement. Gas is tapped from the barrel as the bullet moves past a gas port located above the rifle's front sight base. The gas rushes into the port and down a gas tube, located above the barrel, which runs from the front sight base into the AR-15's upper receiver. Here, the gas tube protrudes into a "gas key" (bolt carrier key) which accepts the gas and funnels it into the bolt carrier.

The bolt and bolt carrier together form a piston, which is caused to expand as the cavity in the bolt carrier fills with high pressure gas. The bolt is locked into the barrel extension, so this expansion forces the bolt carrier backward a short distance in line with the stock of the rifle to first unlock the bolt. As the bolt carrier moves toward the butt of the gun, the bolt cam pin, riding in a slot on the bolt carrier, forces the bolt to turn and unlock from the barrel extension. (The gas system only serves to unlock the bolt once the projectile has exited the barrel). Once the bolt is fully unlocked it begins its rearward movement along with the bolt carrier. The bolt's rearward motion extracts the empty cartridge case from the chamber, and as soon as the neck of the case clears the barrel extension, the bolt's spring-loaded ejector forces it out the ejection port in the side of the upper receiver. The bolt is much heavier than the projectile, and along with the recoil-spring pressure inside the stock buffer-tube performs the cartridge ejection function and chambers the following cartridge.

Behind the bolt carrier is a plastic or metal buffer which rests in line with a return spring that pushes the bolt carrier back toward the chamber. A groove machined into the upper receiver traps the cam pin and prevents it and the bolt from rotating into a closed position. The bolt's locking lugs then push a fresh round from the magazine which is guided by feed ramps into the chamber. As the bolt's locking lugs move past the barrel extension, the cam pin is allowed to twist into a pocket milled into the upper receiver. This twisting action follows the groove cut into the carrier and forces the bolt to twist and "lock" into the barrel's unique extension.

The AR-15 rifle is available in a wide range of configurations from a large number of manufacturers. These configurations range from short carbine-length models with features such as adjustable length stocks and optical sights, to heavy barrel models.

When installing a new complete upper receiver, particularly one designed to handle a different caliber of ammunition (i.e., other than .223 Remington or 5.56x45 mm NATO), some modification to the contents of the lower receiver may be required, depending on the particular conversion. For example, a conversion to 9 mm typically would involve the installation of a magazine well block (to accommodate a typical 9 mm magazine, such as Uzi or Colt SMG), replacing the .223 hammer with one designed for 9 mm ammunition, and depending on the original stock, replacing the buffer, action spring and stock spacer with those designed for the new 9 mm AR-15 configuration. The 9mm cartridge fires from an unlocked breech, or straight blow-back—rather than a locked breech, because the spring and bolt provide enough weight to allow this type of functioning. These guns do not utilize the direct gas impingement method of operation like the original.

Some AR-15's like the POF, LWRCI, H&K, Sturm Ruger, Sig Sauer, and Adams arms offerings replace the DGI (direct gas impingement) operating system with a short stroke/long stroke gas piston system. These guns usually have modified bolt carriers, gas keys, and gas blocks. When fired, DGI systems dump high pressure hot gas through the gas tube to the bolt carrier key and into the bolt carrier group. This can rapidly heat up the bolt carrier group and cause excessive fouling, one of the main complaints about the design. Gas piston operating systems alleviate these problems, but can be the cause of other issues such as bolt carrier tilt.

Some manufacturers offer upper and lower receivers machined from a solid billet (block) of aluminum as opposed to an aluminum forging. These include Double D Armory, LTD, Sun Devil manufacturing, LAR Grizzly manufacturing, POF-USA, and Black Rain. This is usually done for added strength.

Upper receivers utilizing a monolithic rail system that combine a railed hand guard and upper receiver into one uninterrupted piece are made by companies like Colt's Manufacturing Company, Lewis Machine and Tool (LMT MRP), POF-USA, and VLTOR. This is done to provide a continuous uninterrupted rail section that runs along the top of the gun from the weapons charging handle to the front sight/gas block. This rail section is used for the mounting of sights, laser aiming devices, night vision devices, and lighting systems.

A side charging upper receiver has been developed by LAR Grizzly. The charging handle can be had in a left side, right side, or ambidextrous configuration. Since the charging handle is attached to the bolt carrier making it a reciprocating design, it can be utilized as a forward assist device as well.

Early models had a 1:14 rate of twist for the original 55 grain (3.6 g) bullets. This was changed to 1:12 when it was found that 1:14 was insufficient to stabilize a bullet when fired in cold weather. Most recent rifles have a 1:9 or 1:7 twist rate. There is much controversy and speculation as to how differing twist rates affect ballistics and terminal performance with varying loads, but heavier projectiles tend to perform better with faster rifling rates.[19] Additionally, the various non .223 / 5.56 calibers have their own particular twist rate, such as 1:10, 1/11 and 1/12 for 6.8x43mm SPC, 1/10 7.62x39mm, and 1:12 for .308 Winchester.

A Colt AR-15 on display at the National Firearms Museum. This example is fitted with an early waffle-patterned 20-round magazine.

Standard issue magazines are 20- or 30-round staggered-column magazines, traditional box magazines exist in 40- and 45-round capacities, and usable magazines have been constructed from a variety of materials including steel, aluminum, and high-impact plastics. Drum magazines with 90- and 100-round capacities exist, such as Beta C-Mags. Low-capacity magazines, usually of a 5- or 10-round capacity, are available to comply with some areas' legal restrictions, hunting, and because larger magazines can inhibit shooting from a benchrest. Surefire is now offering extended capacity magazines in 60- and 100-round capacity configurations. These magazines are a staggered column design.

Most AR15 rifles have a barrel threaded in 1⁄2-28" threads to incorporate the use of a muzzle device such as a flash suppressor, sound suppressor or muzzle brake.[20] The initial design had three tines or prongs and was prone to breakage and getting entangled in vegetation. The design was later changed to close the end to avoid this and eventually on the A2 version of the rifle, the bottom port was closed to reduce muzzle climb and prevent dust from rising when the rifle was fired in the prone position.[21] For these reasons, the US Military declared this muzzle device a compensator; but it is more commonly known as the "GI" or "A2" flash suppressor.[22]

Some jurisdictions have banned or severely restrict usage of flash suppressors by law. In most of these areas, AR-15 shooters have installed muzzle brakes or compensators on their rifles.

The threaded barrel allows sound suppressors with the same thread pattern to be installed directly to the barrel, however this can result in complications such as being unable to remove the suppressor from the barrel.[23] A number of suppressor manufacturers have turned to designing "direct-connect" sound suppressors which can be installed over an existing flash suppressor as opposed to using the barrel's threads.[23]

AR-15 rifles, like all semi-automatic rifles, are subject to strong restrictions of ownership in all states and territories in Australia. The only means of legally owning an AR-15/M16-type rifle in Australia today beyond law enforcement is to have a Category D Firearms License (e.g. a professional animal culler), to have a Firearms Collector's License and the firearm deactivated (with the barrel plugged up and the action welded shut), or converted to blank fire if one is a member of a military re-enactment organization.

The heavy restrictions on semi-automatic rifles were introduced in 1996 in response to the Port Arthur massacre – one of the firearms used in the attack was an AR-15. Before 1996, AR-15 rifles were legal to own in Queensland and Tasmania.

Strict firearms laws have made the AR-15 rifles too expensive for television and film production with the requirement to destroy or export imported semi-automatic rifles after use. Warwick Firearms & Militaria[24], a Melbourne prop maker, self-manufactures blank firing AR-15-type, WFM4 rifles locally with approximately three dozen having been sold.[25]

In Austria, semi-automatic centerfire rifles have to be classified as sporting or hunting firearms in order to obtain civilian-legal status. After this classification, they are considered "category B" firearms, which means that holders of gun licenses may own them. These licenses are may-issue items if the applicant specifies a valid reason (self-defense at home for example is considered valid by law in any case), passes a psychological test and attends a gun-basics course.
Currently, three AR-15 manufacturers, all producing in Germany have had versions of their AR15 models successfully classified as class B weapons. These Austrian versions differ slightly from the original design in order to ensure that no military full-auto trigger, bolt and barrel may be installed. Additionally, bayonet lugs and flash hiders are prohibited on semi-automatic rifles while Muzzle brakes and compensators are legal. There is no minimum length for barrels, therefore even barrel lengths as short as 7.5" are possible.

The Government of Canada classifies the AR-15 (and its variants) as a restricted firearm. For anyone wanting to lawfully own an AR-15, they must obtain a Possession and Acquisition License (PAL) valid for restricted firearms and then each acquisition of a restricted class firearm is subject to approval by the Chief Firearms Officer (CFO) of the would-be buyer's province of residence.[26][27] With the introduction of strict gun control measures by former Prime Minister Kim Campbell (Bill C-17), the AR-15 had originally been intended to be classified as a prohibited firearm, making it all but impossible to privately own one. However, due to the presence of nationwide Service Rifle target shooting competitions, the AR-15 was granted a sporting exception.

As with all Restricted firearms (including most pistols, some shotguns, and some rifles) AR-15s are allowed to be fired only at certified firing ranges since the CFOs of all provinces and territories have agreed to issue ATTs (Authority To Transport) for these guns only to certified ranges. Since owners can't legally take these guns anywhere else that shooting is allowed, they can in effect only shoot them on certain ranges. In order to legally own and transport a Restricted firearm, the firearm must be registered with the Royal Canadian Mounted PoliceCanadian Firearms Program and must apply for an Authorization to Transport (or ATT) from the Chief Firearms Officer (CFO) for their province or territory. Additionally, the firearm must be unloaded, deactivated by a trigger or action lock, and be in a locked, opaque container during transport.[28]

The issuance of ATTs varies considerably from province to province, and is generally reflective of a particular province's political and social levels of acceptance toward gun ownership. In Ontario the only way to obtain an ATT for restricted firearms is to become a member of a range, whereas in Alberta, where firearms ownership is widely accepted, generally a single ATT is promptly issued that allows citizens to transport firearms to border crossings, gunsmiths, and shooting ranges. Firearms transfers in provinces such as Quebec can take up to 3 months to process.[citation needed]

In Germany and Finland, possession of semi-automatic rifles, including the AR-15, is legal, provided that the rifle's owner acquires a permit for owning one. A license is required for each individual firearm and there needs to be a specific reason for ownership such as participation in the shooting sports and hunting. In Finland maximum magazine capacity in hunting is 3 rounds. But in addition you can have 1 round chambered which brings your direct ammo capacity up to 4 rounds.

For German hunters, their semi-automatic firearm's magazine must be modified in such a way that its maximum capacity is only 2 rounds (excluding handguns), meaning that when hunting game animals only 3 shots in total can be fired (as one additional round is loaded in the chamber) without reloading. This rule is stated in German hunting law and not in German gun law. In fact, magazines are not regulated by German gun law and free to purchase and possess in any capacity and type for anyone.

In Italy, the AR-15 rifle belongs to B7 class and can be owned by civilians, provided it is incapable of fully automatic fire.
Like every other gun, it must be registered and to purchase it citizens must have a valid license, which is granted to every person who qualifies.
Most of the rifles are chambered in .223, but 5.56x45 is also allowed.
Since long arms with a caliber larger than .22 are considered hunting weapons, it is possible to own unlimited AR-15s as they are hunting rifles according to the law.

In Ireland, semi-automatic AR-15s are only legal if they are rim-fire and they have their magazines blocked to 10 rounds. Any other semi-automatic AR requires a restricted license. Non semi-automatic ARs are legal up to .308 and further caliber ARs required restricted firearms licenses.

The AR-15, like all other semi-automatic rifles, is legal for individuals who need one for competitive use (IPSC rifle or 3-gun matches). A valid competition license is required, and all weapons are registered with the police. There are no banned "assault weapon" features or parts. However, the AR-15 is not allowed for hunting use.

The AR-15 like any other semi-automatic long arm in South Africa, is legal for anyone who holds any of the following licenses:

Licence to possess firearm for dedicated hunting and dedicated sports-shooting

Licence to possess firearm for business purposes

Licence to possess restricted firearm for self-defence

While not prohibited common citizens can only own semi-automatic AR-15s if they are members of a hunting or target club, and possess dedicated sport person or dedicated hunter status granted by organisations accredited by the SAPS (South African Police Service).[29][30] Other licenses allowing the possession of semi-automatic rifles are only available to people who require their use in the conduct of their business (e.g. security personnel), and citizens who can convincingly prove to the Registrar that non-restricted firearms are not sufficient to provide protection. The latter requires a specific motivation for the need of a restricted firearm for self-defence[29] and have been granted to rhino farmers.[31][32][33]

As with all semi-automatic, centerfire rifles, AR-15s are classed as a Section 5 weapon, i.e., a person must provide an exceptional reason and gain permission from the Home Secretary, making ownership all but impossible for a private citizen. However, AR-15s in a manually operated straight pull configuration or semi-automatic AR-15s that are chambered to fire a .22 rimfire cartridge are legal and can be held on a standard Section 1 Firearms Certificate. There are no restrictions on 'assault weapon features' in the UK, and no restrictions on magazine capacity. There are a number of UK manufacturers of "straight-pull" AR-15 variants. Southern Gun Company has tried to introduce a 9mm "self-ejecting" variant for gallery rifle shooting nicknamed the "Unicorn" but, despite numerous units being sold on the understanding that the rifle was a compliant Section 1 firearm, the rifles were seized and subjected to stringent testing by the UK Forensic Science Service (FSS). A small number of pre-production models were found to be non-compliant with section 1 status. However, later models were deemed Section 1 compliant and were returned to their owners.[citation needed]

There are no federal restrictions on the ownership of AR-15 rifles in the United States. During the period 1994–2004 variants with certain features such as collapsible stocks, flash suppressors, and bayonet lugs were prohibited for sales to civilians by the Violent Crime Control and Law Enforcement Act of 1994, with the included Assault Weapons Ban. Included in this was a restriction on the pistol grip that protrudes beneath the stock, which was considered an accessory feature under the ban and was also subject to restrictions. Some rifles were manufactured with a grip not described under the Ban installed in its place. Those AR-15s that were manufactured with those features, as well as the accompanying full capacity magazines, were stamped "Restricted Military/Government/Law Enforcement/Export Only". The restrictions only applied to guns manufactured after the ban took effect. It was legal to own, sell, or buy any gun built before 1994. Hundreds of thousands of pre-ban ARs were sold during the ban as well as new guns redesigned to be legal.

Since the expiration of the Federal AWB in September 2004,[34] these features became legal in most states.[35] Since the expiration of the ban, the manufacture and sale of then-restricted rifles has resumed completely.

At least five states (NY, NJ, CA, MD, CT) regulate possession of AR-15 rifles either by the restriction of certain features or outright bans of certain manufacturers' models. For example: the A3 tactical carbine pictured above is legal for sale and possession in the United States generally, but is illegal for sale in California and the Colt AR-15 is banned by name in New Jersey.

Under U.S. firearms laws, the lower receiver of the AR-15 is considered a firearm and subject to purchasing restrictions. (This is not universally the case with rifles. On some other rifles, such as the FN FAL, Heckler & Koch 91, 93, (G-3, G-33), 94, MP-5 or SP-89 (plus clones), the upper receiver is the serial-numbered part, and thus the firearm.) The AR-15 upper receiver assembly is considered a part, and may be purchased and mail-ordered in most locations. This is a desirable feature for enthusiasts, who can purchase a number of upper receivers (often in different calibers and barrel lengths) and interchange them with the same lower receiver.

Adding a shoulder stock to an AR-15 with a barrel shorter than 16" would constitute constructing a Short-Barreled Rifle (SBR) under NFA rules – subject to a $200 tax stamp. The receiver, or serial-numbered part is still considered a firearm, but a receiver has unique status assigned by the Gun Control Act of 1968 as amended, and ATF regulations or rulings. ATF ruling 07-07-2009 illustrates a receiver's unique legal status even if the receiver can only be made into a rifle.[36] Under the United States v. Thompson-Center Arms Company Supreme Court ruling, an individual can possess parts for both the rifle and pistol so long as they are not assembled improperly.[37] This ruling has been further clarified by the ATF Director in a ruling (ATF Ruling 2011-4[38]) dated July 25, 2011 which restates most of the findings in the Thompson case.

Following the 1992 ruling, the ATF claimed that the finding in United States v. Thompson-Center Arms Company only applies to products of Thompson Contender, and not to any other companies' products.[39] This has changed under ATF ruling 2011–4 which states

A firearm, as defined by the National Firearms Act (NFA), 26 U.S.C. 5845(a)(3), is made when unassembled parts are placed in close proximity in such a way that they: (a) serve no useful purpose other than to make a rifle having a barrel or barrels of less than 16 inches in length; or (b) convert a complete weapon into such an NFA firearm.[40] A firearm, as defined by 26 U.S.C. 5845(a)(3) and (a)(4), is not made when parts within a kit that were originally designed to be configured as both a pistol and a rifle are assembled or re-assembled in a configuration not regulated under the NFA (e.g., as a pistol, or a rifle with a barrel or barrels of 16 inches or more in length).[40] A firearm, as defined by 26 U.S.C. 5845(a)(3) and (a)(4), is not made when a pistol is attached to a part or parts designed to convert the pistol into a rifle with a barrel or barrels of 16 inches or more in length, and the parts are later unassembled in a configuration not regulated under the NFA (e.g., as a pistol).[40] A firearm, as defined by 26 U.S.C. 5845(a)(4), is made when a handgun or other weapon with an overall length of less than 26 inches, or a barrel or barrels of less than 16 inches in length, is assembled or produced from a weapon originally assembled or produced only as a rifle.[40] Such a weapon would not be a "pistol" because the weapon was not originally designed, made, and intended to fire a projectile by one hand.

Furthermore, adding a forward pistol grip to an AR-15 designated as a pistol constitutes manufacture of an AOW (any other weapon).[41] Both of these actions require an approved "Form 1" and payment of a $200 tax prior to the actual construction of the item. Current wait times for approval average 5–8 months during which time no modifications or construction may be done.

AR-15 Olympic Arms K8MAG 243 WSSM

As of 2012, there are an estimated 2.5-3.7 million rifles from the AR-15 family in civilian use in the United States.[42] They are favored for target shooting, hunting, and personal protection, and have become the most popular rifle in America.[43]

The Roberti-Roos Assault Weapons Control Act of 1989 banned Colt AR-15 rifles by name in the State of California. California's assault weapons ban following the Supreme Court of California's 2000 decision in Kasler v. Lockyer went further and banned AR-15s made by other manufacturers by name.[44] AR-15-style rifles that are not named specifically by the Roberti-Roos or other restricted lists can be purchased in the state with some minor modifications. Since these are not on the various lists of prohibited firearms, their lower receivers (the part that is legally the firearm) are referred to as "Off List Lowers"(OLL). These OLLs are very common in California, and at least several hundred thousand of them have been sold in the state since the ban went into effect.

With the plethora of manufacturers of complete weapons and aftermarket barrels, there is a potential hazard associated with chamber specifications. Both civilian (SAAMI) specification .223 Remington and 5.56 mm NATO are available. Though the external dimensions of the two cases are the same and both chambers typically accept both types of ammunition, the firing of military specification ammunition in civilian specification chambers can produce chamber pressures greater than the barrel is designed to handle. Internally the 5.56x45mm case wall is thicker, and the round itself is typically loaded to produce higher pressure than the .223. The most common result of firing military 5.56x45mm ammunition in a .223 Remington chamber is that the primer can be forced out of the case by chamber pressure, often resulting in the primer becoming lodged somewhere in the action of the rifle, and disassembly of the rifle is often necessary to remove the jammed primer.[45]

A few AR-15 manufacturers incorporate the use of a hybrid chamber specification known as the Wylde chamber. Designed by and named after Bill Wylde of Greenup, IL, this chambering was designed to accurately shoot the military ball ammo of the day while still feeding reliably. Coincidentally, it shoots the longer 80 gr bullets commonly used in the sport of Highpower Rifle Competition very well and is one of the preferred chambers for that use. While the Wylde chamber allows for optimal seating depth of 80 grain bullets over .223 Remington and 5.56 NATO, it is capable of accepting both ammunition types. The Wylde chamber is used by many manufacturers who sell "National Match" configuration AR-15 rifle, barrels, and upper receivers. The type of chamber, manufacturer, and rifling twist in inches is typically found stamped into the barrel in front of the front sight assembly.
An additional point of concern in the design is the inertial firing pin. A lightweight firing pin rides in a channel inside the bolt unrestrained. When the bolt locks forward during loading, the firing pin typically rides forward and impacts the primer of the chambered round. In military specification ammunition and quality civilian ammunition, this is not normally enough to fire the round and only leaves a small "ding" on the primer. With more sensitive primers or improperly seated primers, this can cause a slamfire during loading.[46]

Another type of malfunction, Hammer Follow, is also a potential problem for AR platform weapons. In one case, David Olofson, an Army Reservist with 16 years in the US Army, lent to a prospective buyer an AR-15 rifle which was found to malfunction in such a way that it would fire multiple rounds on a single trigger pull. After law enforcement intervention, David Olofson was charged with and found guilty of illegally transferring a machine gun and was sentenced to thirty months in prison.[47]

In addition, the AR15 lower receiver can be used as a trigger mechanism for single shot or side-fed upper receiver platforms that shoot in a variety of larger calibers, including .50 BMG and Crossbow bolts.